1. Field of the Invention
The present invention relates to an angular velocity detection apparatus that detects an angular velocity using a surface acoustic wave angular velocity sensor.
2. Description of Related Art
There is known a surface acoustic wave angular velocity sensor for sensing an angular velocity based on a surface acoustic wave (cf. JP-A-H8-334330 and U.S. Pat. No. 6,516,665).
A surface acoustic wave (SAW) angular velocity sensor 2 is described below with reference to FIG. 6, as a related art. The SAW angular velocity sensor 2 includes: a piezoelectric single crystal substrate 3; and multiple perturbation weights 4 located on the substrate 3 and aligned in a square region. The SAW angular velocity sensor 2 further includes a comb electrode 6 for driving use (referred to hereinafter as a driving electrode 6), a comb electrode 8 for sensing use (referred to hereinafter as a sensing electrode 8), and reflectors 7, 9. The electrodes 6, 8 and the reflectors 7, 9 are located in a periphery of the multiple perturbation weights 4.
The driving electrode 6 faces a first side of the square region in which perturbation weights are located. The sensing electrode 8 faces a second side of the square region, which is orthogonal to the first side. The reflectors 7 are located so that the multiple perturbation weights 4 and the driving electrode 6 are between the reflectors 7 in the X-axis direction, as shown in FIG. 6. The reflectors 9 are located so that the multiple perturbation weights 4 and the sensing electrode 8 are between the reflectors 7 in the Y-axis direction, as shown in FIG. 6.
In the above SAW angular velocity sensor, application of a signal (i.e., driving signal) with a resonance frequency (e.g., 10 MHz to several 100 MHz) to the driving electrode 6 leads to generation of a surface acoustic wave (SAW). The resonance frequency is determined from a comb pitch of the driving electrode 6.
The generated SAW is confined by the reflectors 7 such that the SAW propagates back and forth along the X-axis direction in a region between the reflectors 7. Thus, a first standing wave is created in the region between the reflectors 7. A pitch of the multiple perturbation weights is determined in accordance with a wavelength of the first standing wave such that the multiple perturbation weights are located at anti-nodes of the first standing wave. Thus, each perturbation weight 4 vibrates at a position where the first standing wave has maximum amplitude. In the above, “the anti-node of standing wave” means a position where a vibration component perpendicular to a substrate surface becomes maximum.
The multiple perturbation weights 4 are arranged in a zigzag pattern such that perturbation weights 4 are adjacent to each other in a direction parallel to a diagonal of the square region. Perturbation weights 4 adjacent to each other are located at positions where displacements caused by the first standing wave are in anti-phase.
As shown in FIG. 7, the perturbation weight 45 and the perturbation weights 41 to 44 surrounding (i.e., adjacent to) the perturbation weight 45 vibrate in anti-phase. When the driving signal is applied to the driving electrode 6, the first standing wave is generated in the X-axis direction, and the perturbation weight 4 vibrates and has a vibration velocity V in the Z-direction. Under the above state, if the substrate is subjected to an angular velocity Ωx around the X-axis direction, each perturbation weight 4 is subjected to an acceleration a=2V×Ωx due to a Coriolis force, which is proportional to the velocity V of the perturbation weight 4 and the angular velocity Ωx. Since the velocity V and the angular velocity Ωx are vector quantities, a direction and a phase of the Coriolis force acting on the perturbation weight 45 are opposite to those acting on the perturbation weights 41 to 44 surrounding the perturbation weight 45. Note that the vector variables are shown as bold characters (e.g., Ωx) in the above notation.
The Coriolis force generates the vibration force acting on each perturbation weight in the Y-axis direction. The vibration force causes an acoustic wave in the Y-axis direction, which is orthogonal to the first standing wave in the x-axis direction. Since the perturbation weights 4 (e.g., the perturbation weights 41 and 42) are spaced apart from each other in the Y-axis direction by an integral multiple of the wavelength of the acoustic wave, and since the acoustic wave propagates back an forth in a region between the reflectors 9, a second standing wave is created in the region between the reflectors 9.
A magnitude of the second standing wave is proportional to the Coriolis force. Thus, to sense the angular velocity by using the SAW angular velocity sensor 2, one can apply the driving signal to the driving electrode 6 and measure an output (e.g., voltage, electric charge) from the sensing electrode 8.
The inventors clarify difficulties associated with the above SAW angular velocity sensor. The difficulties are described below, as a related art. According to a principal and a configuration of the above SAW angular velocity sensor 2, the first standing wave generated by the driving electrode 6 are scattered at the perturbation weights 4. Thus, even in a steady state with no angular velocity being applied, the sensing electrode 8 senses the scattered wave.
The scattered wave has a maximum displacement at a position of the perturbation weight 4 at a time when the perturbation weight 4 has a maximum displacement due to the first standing wave. Thus, the scattered wave and the first standing wave vibrate in phase. On the other hand, the second standing wave due to the Coriolis force has a maximum displacement at a time when the perturbation weight 4 has a zero displacement and a maximum vibration velocity due to the first standing wave. Thus, a phase of the second standing wave is different in generally 90 degrees from that of the first standing wave.
According to an idea of measuring an angular velocity using the SAW angular velocity sensor 2, a signal component representative of the Coriolis force or an angular velocity is extracted by synchronously detecting an output signal from the sensing electrode 8 by using a reference signal that corresponds to the driving signal.
The inventors however have found that, when an angular velocity is measured by synchronous detection utilizing a SAW angular velocity sensor, a detection result has a large error. Further, the inventors have revealed that reasons for the large error include the followings.                (1) The scattered wave propagating to the sensing electrode 8 is larger than a detection target wave due to Coriolis force by 6 orders of magnitude in voltage equivalent.        (2) Some vibration components directly propagate from the driving electrode 6 to the sensing electrode 8 (i.e., direct arrival wave). The direct arrival wave is larger than the detection target wave due to Coriolis force by 5 orders of magnitude in voltage equivalent at a portion where a distance between the driving electrode 6 and the sensing electrode 8 is minimum (e.g., 0.1 mm).        
That is, in the SAW angular velocity sensor 2, an unwanted wave such as the scattered wave and the direct arrival wave becomes much larger than the wave due to the Coriolis force. In the above case, if a synchronous detection circuit is used for removing an unwanted wave signal component from the output signal of the sensing electrode 8, the synchronous detection circuit is saturated, and thus, the unwanted wave signal component cannot be removed.